It all depends on the volume of air space in the room, the rooms shape and floor to ceiling height etc. There are a lot of variables and that science of fluid dynamics comes into play! If you go for a very powerful fan you may find it extracting all the room heat outside and causing excessive draughts, unless you opt for a fan with a heat recovery system.
I fitted a "Belling" chimney style hood in my kitchen and find it very good. I kept the extract point as close to the fan as possible, keeping the ducting less than 2 foot in length. Long lengths of ducting, corners and turns will increase the internal surface area and friction within the ducting and this will dramatically affect the performance of any fan. It's important to have sufficient air coming into the room to replace the air being extracted- otherwise the fan will labour (air starvation) and performance will drop considerably. I have two 6" wall vents in my kitchen (standard grant sized house). A fan with a rating of 250- 300 watts should be adequate for most domestic kitchens.